usb: isp1760: Remove duplicate usb_disabled() check

Both isp1760_register() and isp1761_pci_probe() check whether USB is
disabled by calling usb_disabled(), and bail out with an error if it is.
One check is enough, remove the PCI-specific check.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>